How to modify a model for Unity?
Alright, i'll be as detailed as possible.

Okay so first, create your model, for demonstration purposes, I have a cube.



After creating your model, select all the faces in edit mode and press U then select whatever option is best for you. I used the normal "Unwrap".



After that, create your texture, and apply it to the model, to do that, click New or Open in the UV/Image editor (while in edit mode and with your models faces selected).

I clicked new and in the menu it opens I set the image from Blank to Color grid.



Now that that's all done, we've got our texture, time to save it. Do that by clicking Image (It should have a little * at the end of it) and click "Save As Image", and select your location, can be anywhere.


(set the render view to Textured to see what it looks like)

And now, finally save the model as well. You should use a .blend as the format, but some other formats may work too, but for the sake of a tutorial, you should use .blend.

Alright, so next, drag your model into the Project in Unity, do that by simply dragging and dropping the file from the location you saved it to, into Unity's Project window.



And now, drag it into the Hierarchy, it should be in the scene now.



Nice, we've got a model imported into unity now. Okay so now we do the dragging and dropping of the texture in the project window as well, like we did with the model.

After that's put into the Project window alike the model, we will then drag it onto the object.



Nice, we've got our model all textured and ready to go, and all set up. And now time to make some scripts so it does something. :)

Good luck!